Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as ultraviolet light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask stage for aligning the mask, a projection objective (often simply referred to as “the lens”) and a wafer stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens, for which usually |β|<1 holds, for example |β|=¼ or |β|= 1/100.
An aim in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to an increased output of the manufacturing process and also to a high integration density. This, in turn, has usually a favorable effect on the performance of the produced microstructured components.
The minimum size of the structures that can be lithographically defined is approximately proportional to the wavelength of the projection light. Therefore the manufacturers of such apparatus strive to use projection light having shorter and shorter wavelengths. The shortest wavelengths currently used are 365 nm, 248 nm, 193 nm and 13 nm and thus lie in the deep (DUV), vacuum (VUV) or extreme (EUV) ultraviolet spectral range, respectively.
The correction of aberrations is becoming increasingly important for projection objectives designed for operating wavelengths in the DUV and VUV spectral range. Different types of aberrations usually involve different correction measures.
The correction of field independent and rotationally symmetric aberrations is comparatively straightforward. A field independent aberration is referred to as being rotationally symmetric if the wavefront deformation in the exit pupil of the projection objective is rotationally symmetric. The term wavefront deformation refers to the deviation of a wavefront from an ideal aberration-free wavefront. Rotationally symmetric aberrations can be corrected, for example, at least partially by moving individual optical elements along the optical axis.
Correction of those aberrations which are not rotationally symmetric is more difficult. Such aberrations occur, for example, because lenses and other optical elements heat up rotationally asymmetrically. One aberration of this type is astigmatism.
In order to correct rotationally asymmetric aberrations, U.S. Pat. No. 6,338,823 B1 proposes a lens which can be selectively deformed with the aid of a plurality of actuators distributed along the circumference of the lens. The deformation of the lens is determined such that heat induced aberrations are at least partially corrected. A more complex type of such a wavefront correction device is disclosed in US 2010/0128367 A1.
US 2010/0201958 A1 and US 2009/0257032 A1 disclose a wavefront correction device that includes two glass plates that are separated from each other by a liquid. A corrective effect on the optical wavefront is not obtained by deforming the glass plates, but by changing their refractive index locally. To this end one of the two glass plates is provided with heating wires that extend over a surface through which projection light passes. With the help of the heating wires a temperature distribution inside the glass plate can be produced that causes, via the dependency dn/dT of the refractive index n on the temperature T, the desired distribution of the refractive index.
U.S. Pat. No. 6,504,597 B2 and WO 2013/044936 A1 propose correction devices in which heating light is coupled into a lens or a plate via its peripheral rim surface, i.e. circumferentially. Optical fibers may be used to direct the heating light produced by a single light source to the various locations distributed along the periphery of the optical element.
WO 2011/116792 A1 and EP 1 524 558 A1 disclose a wavefront correction device in which a plurality of fluid flows emerging from a corresponding number of fluid outlet apertures enter a space in the vicinity of a pupil plane through which projection light propagates during operation of the projection exposure apparatus. A temperature controller, which may include heat dissipating members that are arranged at the outside of channels walls, sets the temperature including individually for each fluid flow. The temperature distribution is determined such that optical path length differences caused by the temperature distribution correct wavefront errors. An advantage of this prior art wavefront correction devices is that it is able to correct also wavefront deformations which change very quickly, for example during a single scan cycle. However, it is difficult to produce a laminar fluid flow by combining the plurality of individual fluid flows.